In an image forming apparatus forming a toner image using an electrophotographic method, when an environmental condition such as temperature and humidity changes or when a continuous printing operation is performed for a long time period, a toner adhesion amount relative to a toner image per unit area may change, so that the image density varies. In a color image forming apparatus forming a color image, when the toner adhesion amounts of plural primary colors fluctuate (change), the color tone of the corresponding superimposed color (e.g., a combination of the L* value, the a* value, and the b* value in the L*a*b* color coordinate system) may be disturbed (influenced). Specifically, the color(s) reproduced in a color image forming apparatus is roughly classified into the primary color and the superimposed color. The primary color refers to a color that is expressed by using a single kind of toner. For example, in a configuration where there are four kinds of colors which are yellow (Y), magenta (M), cyan (C), and black (K), the color expressed by using only any one of the Y, M, C, and K toners is the primary color. On the other hand, the superimposed color refers to a color which is expressed by using two or more primary colors. Namely, the superimposed color is reproduced by superimposing plural primary colors. Therefore, when the toner adhesion amounts of the corresponding primary colors fluctuate (change), the color tone of the corresponding superimposed color reproduced by superimposing the primary colors may be disturbed (influenced).
Therefore, Patent Document 1 describes a color marking device that calibrates color adjustment by outputting a test pattern of the superimposed colors and multiple tones on a test print sheet and estimating the density based on the data of the reflectance of the test print sheet, so as to control the image processing conditions of the image density and the like. Namely, specifically, plural test patterns for the calibration to determine an image forming parameter, specifically a tone reproduction curve, are formed on a test print sheet besides the print sheets on which the images based on the user's instructions are formed. Then, the L* value, the a* value, and the b* value of the color references of the test patterns are detected, and based on the detected result, the tone reproduction curve is corrected. After that, based on the corrected tone reproduction curve, the color toner image of the superimposed colors and multiple tones is formed. By doing this, when a state of the image forming process is changed, the fluctuation of the colors output on the sheets may be controlled, and stable image quality may be obtained.
On the other hand, recently, there have been developed color production printers that quickly output a larger amount of color documents such as flyers, catalogs, reports, bills and the like. Such color production printers are typically used so that, for example, tens of millions of telephone bills and receipts are issued within one week or so. In this case, the printing is continuously performed during all day and night in a period such as one week. In other words, hundreds of sheets per minute are continuously printed for tens of hours. Due to this requirement, the color production printers have the characteristic (requirement) that it is absolutely not possible to stop the apparatus during the continuous operation. This is because, by stopping the apparatus, the necessary large amount of sheets may not be printed before the due date of issue. In this regard, the fast-printing type color production printers are technically much different from the printers (MPFs: Multifunction Peripherals) that may be installed in offices.
When the method of controlling setting values representing the tone reproduction curve of the image processing parameter described in Patent Document 1 is used, besides the print sheets on which images based on the user's instructions are output (printed), it is necessary to additionally print (discharge) the test print sheet on which the test patterns described above are output. Therefore, the user has to separate the test print sheet from the print sheets. This separating operation is an extremely troublesome task for the user. Therefore, it may not be practical to employ such a configuration to output the test toner image. Accordingly, it may not be possible to frequently perform the control of setting values representing the tone reproduction curve as described in Patent Document 1. Especially, in the case where hundreds of sheets per minute are quickly and continuously printed for tens of hours as the fast type color production printers described above, it may become necessary to stop printing every several minutes to control the setting values representing the tone reproduction curve. As described above, this is contrary to the characteristic of the fast type color production printer that it is absolutely not possible to stop the apparatus during the continuous operation. Further, when the sheets are continuous printed without performing the control of the setting values representing the tone reproduction curve, the state of the process may be largely changed, and the image quality may be degraded. Namely, regarding the fast type color production printer, a new configuration may be necessary that always performs the control of the setting values representing the tone reproduction curve in real time without stopping the printing operation.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2002-033935    [Patent Document 2] Japanese Laid-Open Patent Application No. 2004-229294